The contract is that the method is only invoked when there are no elements on
the prototype, and this elements type forbids accessor elements. So it is safe
to limit the search to the end of the backing store.

BUG=chromium:634269, v8:5162
R=cbruni@chromium.org, mstarzinger@chromium.org

Review-Url: https://codereview.chromium.org/2209273002
Cr-Commit-Position: refs/heads/master@{#38344}